Continuously operating inkjet printing devices have been in commercial use for many years for labeling a large variety of products. In general, a technical distinction may be made between two variants having the same principle of operation, namely, the single-jet continuous printer and the multijet continuous printer.
The common principle of operation lies in the fact that an ink to be printed is conveyed at positive pressure via pumps from a supply container into a pressure chamber having a nozzle, in the actual print head. The nozzle may have an opening diameter in the range of 30 pm to 200 pm, for example. The ink jet is emitted from the nozzle, for example, initially as a continuous ink jet. This, however, is impractical for labeling because the characters produced by this method are composed of single dots, i.e., individual ink droplets. In order to disperse the ink jet into individual homogeneous ink droplets, a modulation element is provided at the pressure chamber which generates pressure fluctuations in the exiting ink jet, so that after being emitted from the nozzle the ink jet is dispersed into individual homogeneous ink droplets. Shortly before the ink droplets separate they are each provided with an individual electrical charge, the magnitude of the charge depending on the desired position of impingement on the product to be labeled.
It is also possible to use pulsed print heads which instantaneously generate individual ink droplets which may be charged or which are already charged.
To ensure the charging, the ink has a low electrical conductivity. On their initially linear course on an original trajectory, the electrically charged ink droplets enter the electrostatic field of a plate capacitor and, depending on their individual charge, are more or less deflected from their linear motion, and after leaving the electrostatic field they continue at a specified angle, which is a function of the charge, with respect to their original trajectory.
By use of this principle, various impingement positions may be selected on a surface to be labeled with individual ink droplets, this occurring only in one direction of deflection. To mask individual droplets from the typeface, or if the droplets are not to be printed, the ink droplets acquire a certain charge or remain uncharged, so that after exiting the electrostatic field of the plate capacitor they strike a collection tube from which they are pumped back to the ink tank. In this manner the ink circulates in the system in a circuit, which has led to the term “continuous inkjet printer.”
In the second referenced system, the multijet printer, there are multiple nozzles from which ink droplets are emitted simultaneously, wherein it is possible to individually charge the single droplets in each jet. In this case, however, only two charge states are produced by which the droplets either strike the respective collection tube or strike a designated fixed position on the surface of the product to be labeled.
By operating multiple nozzles simultaneously and positioning the nozzles in a line in this case, horizontal lines of a character may be controlled in parallel, not in series as in the single jet continuous printer, thereby significantly increasing the speed. A disadvantage of the multijet continuous printer system is that only a fixed number of addressable positions, corresponding to the number of available nozzles, may be addressed.
A disadvantage of the single jet continuous printer design is that the trajectories of the ink droplets emitted from the print head diffuse in a fan-like pattern, so that the font size of a character changes with the distance between the surface to be labeled and the print head, and legibility is impaired as the distance increases.